The American Heritage® Dictionary of the English Language, 4th Edition
adj. Relating to, having, or being a tail fin in which the upper lobe is larger than the lower and the vertebral column extends into the upper lobe, as in sharks.
adj. Of a fish's tail, such that the vertebral column bends and extends upwards into the upper lobe of the tail, making it larger than the lower lobe; as in sharks, rays, and skates.
the GNU version of the Collaborative International Dictionary of English
adj. Having the vertebral column evidently continued into the upper lobe of the tail, which is usually longer than the lower one, as in sharks.
The Century Dictionary and Cyclopedia
In ichthyology, having an unequally divided tail or caudal fin. Contrasted with homocercal.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. possessing a tail with the upper lobe larger than the lower and with the vertebral column prolonged into the upper lobe
Word Usage
"This so called heterocercal tail is only known to exist, as a permanent adult feature, in the sturgeons of to-day."